Inverse emulsion polymers are commonly used for a variety of industrial processes. For example, polymers in this form such as polyacrylamides, and related copolymers, are used as friction reducers to improve fluid flow and as flocculants to enhance the rate of separation of solids from liquids. Processes such as water clarification, industrial and municipal sludge dewatering, papermaking, mineral processing and tailings treatment use inverse emulsion polymers as solid-liquid separation aids.
As used herein, the term “inverse emulsion” refers to an aqueous (water) phase dispersed in a non-aqueous (organic or oil) phase, where the aqueous phase and oil phase are, respectively, the discontinuous and continuous phases. In such emulsions, polymer molecules such as friction reducers or flocculants can be packed inside aqueous phase droplets that are emulsified in the oil phase. The inverse emulsion (active) polymers are coiled within the water phase of the inverse emulsions, but before the active emulsion polymer can be used, the emulsion must undergo inversion so that the polymer is released. The inverse emulsion form of the polymers facilitates the handling, transport, and metering of the liquid active polymer into a process, and the inversion of these emulsions typically produces an aqueous solution that can be ready to use without excessive mixing or solution aging time. A high rate of inversion and high extent of inversion of these polymer emulsions are desirable features, to yield a dissolved polymer solution that is capable of performing solid liquid separations with maximum efficiency.
Inverse emulsion polymers can be prepared by emulsification of a water-soluble monomer in the oil phase, with subsequent polymerization, a process called inverse emulsion polymerization. In an inverse emulsion polymerization, a hydrophilic monomer or blend of monomers, frequently in aqueous solution, is emulsified in a continuous oil phase using water-in-oil emulsifiers and polymerized using either an oil-soluble or water-soluble initiator. A water-in-oil emulsion results, typically a viscous liquid formed from submicroscopic, water-containing, hydrophilic polymer particles suspended in the continuous oil phase.
Surfactants can be used to provide stability to the resulting emulsion. Then, to invert the emulsion, the phases are reversed so that the active emulsion polymers can be released from the discontinuous aqueous phase. When the inverse emulsion is added to a large volume of an aqueous solution, there is a disruption of the previously-dispersed aqueous droplets, allowing the active emulsion polymers contained therein to be released within the aqueous solution where they produce their desired effects. This process whereby the phases of the inverse emulsion are reversed is termed “inversion.” Inversion is facilitated by the addition of surfactants, termed breaker surfactants, that help to disrupt the stability of the original inverse emulsion when it is dispersed within an aqueous solution. Exemplary processes are described, for example, in U.S. Pat. No. 7,429,625 and U.S. Pat. No. 4,525,496.
To optimize the effectiveness of active emulsion polymer contained in an inverse emulsion, it is important that the inverse emulsion can be inverted quickly, thereby releasing the active emulsion polymers into a continuous aqueous phase. It may be difficult to accomplish this, however. For one reason, the surfactants that are used in forming inverse emulsions tend to make the water-in-oil emulsion highly stable, so that it resists inversion. Upon inversion of the emulsion, the polymer chains need to become dissolved, hydrated, uncoiled, or disentangled in order to make the polymers available to perform as flocculants or friction reducers. As an additional problem, the aqueous solution into which the emulsion is inverted can have a high salinity, which hinders the egress and hydration of the polymers from the discontinuous aqueous droplets in the original emulsion. In some instances, the availability of dilution water with sufficient quality, such as a low concentration of dissolved salts, is limited. When the polymer emulsion is inverted in water containing high levels of dissolved salts, and in particular high concentrations of polyvalent salts, the resulting polymer solution can have a diminished viscosity and diminished performance as a solid-liquid separation aid. In other instances, active polymer emulsions are added directly to a process stream to dilute the polymer in the process itself. There remains a need for improved compositions and methods for fast-inverting liquid polymer emulsions, and especially for polymer emulsions that invert quickly and completely in waters containing dissolved salts. With the rapid release of the friction-reduction polymers into the aqueous solution, that is, with the rapid inversion of the inverse emulsion that contains them, the rheological effects of these friction-reducing agents can be rapidly achieved.
There remains a need in the art, therefore, for formulations and methods that allow the formation of inverse emulsions containing polymers in the discontinuous (aqueous) phase, with the rapid inversion of such emulsions upon dispersion into a dominant aqueous solution.